1. Field of the Invention
The invention relates to a fuel cell system and a control method thereof.
2. Description of the Related Art
A fuel cell stack is formed by stacking a plurality of unit cells. Each of the unit cells is configured such that a membrane electrode assembly (hereinafter, referred to as MEA) is sandwiched by two separators. The MEA is formed from an electrolyte membrane in which platinum as a catalytic electrode is applied to both surfaces thereof, and a pair of gas diffusion electrodes which sandwich the electrolyte membrane. The catalytic electrode and the gas diffusion electrode on one surface of the electrolyte membrane form an anode, and the catalytic electrode and the gas diffusion electrode on the other surface form a cathode. A fuel gas passage through which hydrogen gas as fuel gas is distributed to a unit cell is formed in the separator that faces the anode, and an oxidizing gas passage through which air as oxidizing gas is distributed to the unit cell is formed in a separator that faces the cathode. Also, based on the fact that the electrolyte membrane normally exhibits good proton conductivity in a humid state, fuel gas and oxidizing gas are supplied after being humidified such that the electrolyte membrane is maintained to be humid.
In a fuel cell stack of this type, when a circuit which connects an output terminal of a fuel cell and a load is opened, a phenomenon in which the hydrogen gas on the anode side passes through the electrolyte membrane and reaches the cathode side as it is without being protonated, that is, so-called cross leak is caused. As a result, there is a possibility that sufficient durability cannot be obtained due to the cross leak of the hydrogen gas. For example, when the amount of the cross-leaked hydrogen gas is large, hydrogen directly reacts with oxygen on the cathode side, which generates a large amount of heat, and the electrolyte membrane is damaged due to the heat. As a result, there is a possibility that sufficient durability cannot be obtained. Therefore, in order to prevent such cross leak of the hydrogen gas, a method is conceivable, in which the hydrogen gas that remains in each of the unit cells is consumed by short-circuiting a positive electrode and a negative electrode of the output terminal after the supply of the fuel gas is stopped.
However, normally, distribution of the fuel gas to each of the unit cells is configured such that the fuel gas is supplied from a fuel gas supply source to a fuel gas supply manifold of the fuel cell stack through gas piping, and the fuel gas is distributed from the manifold to each of the unit cells. Also, since interruption of the supply of the fuel gas is configured to be performed by a valve or the like which is provided in the gas piping, a considerable amount of the fuel gas remains in the gas piping and the manifold after the supply of the gas is interrupted. When the positive electrode and the negative electrode of the output terminal are short-circuited in this state, a large amount of heat is generated. Accordingly, there still exists a problem concerning durability. Also, there is another problem that it becomes difficult to obtain good fuel economy, which is supposed to be obtained, since a large amount of fuel gas is unnecessarily consumed.
A configuration in which a positive electrode and a negative electrode of an output terminal are short-circuited when operation of a fuel cell is stopped is disclosed in Japanese Patent Laid-Open Publication No. 9-139221. However, an object thereof is to eliminate residual voltage so as to prevent electric shock which is caused during a checkup after the operation. Accordingly, prevention of the cross leak of hydrogen gas, and the fuel economy are not considered.